Re: 3 disk RAID1?

[Mark Knecht <markknecht@gmail.com>]

On Mon, Mar 8, 2010 at 12:58 PM, Robin Hill <robin@robinhill.me.uk> wrote:
> On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:
>
>> Hi all,
>>    I'm still very much on a steep learning curve about what I can do
>> with Linux software RAID. In another thread this weekend a couple of
>> responders discussed among themselves 3-disk RAID1 solutions that can
>> survive if 2 disks die. I don't understand what that means. Can
>> someone point me at a quick explanation? Is that really possible?
>>
>>    In general I'm using a few Wikipedia pages and gravitate toward the
>> diagrams as much as anything.
>>
>> http://en.wikipedia.org/wiki/RAID
>> http://en.wikipedia.org/wiki/RAID1#RAID_1
>>
>> RAID0 - striping, speed not reliability (2 disk minimum)
>> RAID1 - duplicate data, no other protection (2 disk minimum)
>>
>>    How do I build RAID1 using three drives? Just duplicate the data 3
>> times? If drives start going bad how do I determine which one or two
>> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
>> straightforward to figure out, but 2?
>>
> A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
> against hardware failures, so you know which disk has failed because it
> gets kicked out of the array as faulty.  This is the same regardless of
> how many mirrored copies you have (md will detect a write failure to a
> drive and mark it as faulty - read errors will cause the failed block to
> get rewritten).
>
> As for how to create it - it's just the same process as for a 2-disk
> RAID1 but specifying 3 drives (assuming you're using Linux md software
> RAID - if not, please specify what you're intending to use).  The manual
> page for mdadm should give you everything you need - do ask if there's
> anything you want clarifying though.
>
> Cheers,
>    Robin

Thanks Robin. Maybe I am getting smarter about this if I'm figuring
out what others are talking about! ;-)

Cheers,
Mark
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